This invention relates to a method and an apparatus for controlling PWM (pulse width modulation) inverters used for driving an AC motor such as an induction motor and particularly to PWM inverter control method and apparatus directed to suppress oscillations of the AC motor caused when the AC motor is driven at varying speeds under a constant voltage/frequency ratio.
When the PWM inverters are used to drive the induction motor, open loop control is generally employed to make the voltage/frequency ratio constant. In an induction motor with a comparatively low moment of inertia, an unstable phenomenon responsible for occurrence of abnormal oscillations takes place when the induction motor is driven at frequencies, for example, 10 to 30 Hz which are lower than a rated frequency under a low load conduction which ranges from no load to about 10 to 20% loading.
A known method for suppressing the abnormal oscillations of the motor due to the unstable phenomenon is described in U.S. Pat. No. 3,800,199 (corresponding to Japanese Patent Publication No. 27807/78). According to this known method, a signal tuned to an oscillation frequency band of the rotor is extracted from a power amount supplied to the motor and detected through a band-pass filter, and the primary frequency is corrected by the tuned signal to suppress the oscillations.
The funed signal, however, changes with capacity of the motor and even for motors of the same capacity, it changes with the number of poles of the motor and the magnitude of load on the motor. Therefore, there needs a practically unacceptable setting of constants of the band-pass filter for one motor to another. In addition, when motor loading deviates from a design load, stability is disturbed. For these reasons, the known method fails to have applicability to general-purpose inverters for driving unspecified motors.